Lightroom's adjustments cope better with poorly exposed images than Aperture's. Both applications can rescue highlights and shadows to the same degree but the results in Lightroom look nicer. If your images are well exposed to begin with, I find processing results to be about equal.

Lens corrections are very nice and extremely useful. The sharpening workflow is powerful and has better visual feedback to guide your settings than in Aperture.

Lightroom's organisation sucks. Aperture's Project metaphor is far more powerful than Lightroom's folder based approach. Aperture's smart folders are also better, with more relevant criteria available.

In Aperture's workflow, the image comes first and you decide which process you want to apply (organise, post-process, export etc) whereas in Lightroom's workflow, the process comes first and is then applied to the images (and the interface is often specific to the process). In practice I have found Aperture's workflow far simpler and switching between processes mid-flow (say jumping from editing to key wording to geotagging and back) much easier. Lightroom imposes a modular workflow and Adobe intends photographers to work sequentially through importing to editing and finally publishing their images, which I've found works fine until you need to break that sequence and you are then let down by poor interface design that hampers your ability to quickly jump between modules without losing your place.

I switched last year due my Fuji XTrans files. Totally agree with the above. On my 2008 2.8 iMac, Aperture appears more fluid in its general execution. I assume its better integrated into OSX and, irrespective of cpu usage, runs better. But LR is an excellent editor.